1. Field of the Invention
In general, the present invention relates to systems and methods for determining the concentration of impurities in a sample of gas. More particularly, the present invention relates to systems that detect the concentration of impurities within a sample of hydrogen gas.
2. Prior Art Description
In industry, there are many applications for the use of pure or ultra pure hydrogen. For instance, there are many fuel cells that require the use of hydrogen as fuel. The hydrogen, however, must be pure or ultra pure. Molecules of carbon monoxide, hydrocarbon gases, ammonia, or sulfur compounds can cause damage to the fuel cell and decrease both the efficiency and the functional life of the fuel cell.
Ultra pure hydrogen is also used in the manufacture of printed circuits. If the hydrogen gas is contaminated even with a small amount of a contaminant, such as water vapor, the operational integrity of the circuits can be compromised.
Traditionally, pure and ultra pure hydrogen gas is generated using a two-stage process. In the first stage, hydrogen gas is separated from a source gas. For example, hydrogen can be extracted from either a hydrocarbon or water. However, the extracted hydrogen gas produced is not pure. Rather, when hydrogen is extracted, the resultant gas is often contaminated with hydrocarbons, sulfur compounds, water vapor and/or other gases. It is for this reason that a second processing stage is used.
In the second processing stage, the extracted hydrogen gas is purified to reduce contaminants to or below the maximum specified. In the art, pure hydrogen is commonly considered to be hydrogen having purity levels of at least 99.95% and ultra pure hydrogen is commonly considered to be hydrogen having purity levels of at least 99.99999%. In the prior art, one of the most common ways to create pure hydrogen gas is to pass the gas through a bed of adsorbent material that strips the contaminants from the gas. One of the most common ways to produce ultra-pure hydrogen is to pass the gas through a hydrogen separator that contains a membrane made of a hydrogen permeable material, such as palladium or a palladium alloy. When the hydrogen gas is exposed to the hydrogen permeable membrane, and the partial pressure of hydrogen is higher on one side of the membrane, the hydrogen passes through the membrane from the side where the partial pressure of hydrogen is higher to side to the hydrogen lower pressure is lower. The contaminants do not pass. The hydrogen that passes through the membrane, therefore, becomes purified and is collected for use.
Even after hydrogen gas is purified and collected, there are many ways that the purified gas can again become contaminated. The purifier being used may fail and start to pass impure hydrogen. Furthermore, downstream piping may be contaminated causing the gas to become impure as it passes through the piping. The purifier or downstream piping may also develop leaks due to ware, material defects, ground movements and/or many other reasons. In critical applications, where the purity of the hydrogen gas must be maintained, the application must be monitored for small absolute changes in gas purity before the impurities become large enough to degrade the process and or product.
If a manufacturer does not continuously monitor the purity of the pure or ultra pure hydrogen being used, a contaminant leak could destroy fuel cells, ruin microcircuit production, or otherwise cause harm to a product or manufacturing process. In order to prevent such damage from occurring, many manufacturers periodically or continuously measure the level of contaminants in the ultra pure hydrogen and apply statistical process controls to the collected data to predict when a hydrogen purifier needs to be replaced. In order for statistical process controls to be effectively used, very small increases in contaminants need to be detected and tracked.
In the prior art, pure hydrogen and ultra pure hydrogen are typically tested for purity using a gas chromatograph, a Fourier transform infrared (FTIR) detector, gas chromatography (GC) with a variety of detectors, or a mass spectrometer. In all of these techniques any sample of hydrogen gas can have 4 orders of magnitude more hydrogen than the harmful contamination that is to be measured. By the time the sample is ready for analysis, the amount of contaminants may be reduced so significantly that the contaminants may not be quantifiable.
Contaminants can be concentrated by removing some of the hydrogen in a collected sample using a hydrogen permeable membrane. Ultrapure helium can then be added as the carrier gas to transport the contamination to an appropriate detection device. Such a system is exemplified in U.S. Pat. No 5,360,467 to Katkar. However, the concentrated sample must be carried to the measurement device with an ultrapure stream of helium. The helium has its own contamination levels, that need to be separated from the contamination originally in the hydrogen stream. The contamination at this point, after adding helium as a carrier gas, still makes up a very small fraction of the gas stream to be analyzed. The partial pressure of the contamination gases, such as CO and H2S, make up a very small portion of the gas sample, typically less than 1 part per million.
With the increasing popularity of fuel cell technology, many small companies now have the need to test for contaminants in hydrogen gas. For example, some gasoline stations now provide hydrogen gas as fuel for fuel cell powered cars. By regulation, the hydrogen gas must be periodically tested for purity. Such testing must be outsourced to labs because there are no techniques available to measure the carbon monoxide and sulfur contaminations on site. Lab results take time to receive. Accordingly, a gas station may be selling contaminated gas for days, weeks or months before the problem can be detected due to degradation of the fuel cell. The targeted useful life of an automobile fuel cell is greater than ten years if the hydrogen is used for the fuel cell always meets the purity requirements. The dynamic range of an instrument meeting this need will be approximately six orders of magnitude in terms of impurity concentrations across gas types with a resolution for each contamination gas of approximate 1% of the maximum allowed impurity level.
A long standing need, therefore, exists for a system that can sample hydrogen gas and quantify various contaminating gases with widely ranging contamination levels in a simple and cost effective manner. This need is met by the present invention as described and claimed below.